Midprecordial leads V2-V4 (sometimesV1-V5) often show nonspecific or innocent ST junctional to segment elevation, 0.4mV high at times. We would like to make it clear the underlying mechanism for it by following observations and experiments.1. The lead of maximal ST elevation over midprecordium coincided geographically with the degree of horizontal plane rotation as determined by echocardiography and/or angiocardiography in patients with MCLS pulmonic stenosis atrial septal defect ventricular septal defect and tetralogy of fallot.2. Electrical potential distrbution of the ventricular septum at ST segment which is determined by both general or common action potential contour and propagation of whole human ventricular excitation by Sodi-Pallares indicates that ST elevation should be depicted on the lead where the septum coalesces with the anterior free wall. It is because excitation propagates from anteroinferior to posterosuperior along the septum and the lead on the place where the sept
… Moreum coalesces with free wall views and records the potential distribution of the septum most directly from precordium. On the other hand because V3R and V6 are perpendicular to the septum, this potential distribution does not contribute to the net ST potential in these leads.3. Epicardial potential distribution maps on 6 canine experiments revealed maximal ST elevations along the region of left anteror descending coronary artery along the course of which the septum is known to coalesce with the free ventricular wall. This observation supports the idea that the area views the potential distribution longitudinally backwards from the free wall where the septum coalesces with it.4. We obtained similar observations on 16 congenital cardiac patients at surgery. Epicardial potential maps from these patients revealed maximal ST elevation along LAD coronary artery as well with gradual decay of elevation sidewise peripherally.These observations indicate that non-pathological ST elevation over midprecordial leads reflects and records the potential gradient progression along the septum by wiesing posterioly, directly from the place where the septum coalesces with free wall. Less